5th quadrennial Conference of the Royal Institute of Navigation: Birds, Humans & Other Animals

J.F. Oliveira, O.C. Alves, D.M.S. Esquivel , Michael Winklhofer and E. Wajnberg It has been observed that the building nests and feeding activity of some termite species are affected by the geomagnetic field1. The Neocapritermes opacus termite is the only prey of Pachycondyla marginata ants, which displays a migratory behavior2. The migration is significantly oriented at 13o relative to the magnetic North-South axis3. To explore the biophysical mechanisms underlying magnetoreception in termites, the nature of their magnetic sensory system has to be clarified first. Following the widely accepted hypothesis that biomineralized magnetic nanoparticles form part of the magnetoreceptor cells, we conducted a series of low-temperature magnetic measurements to characterize the magnetic material in termites. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves are particularly suitable to identify magnetic nanoparticles. First, a sample is cooled to liquid helium temperatures under zero applied field (ZFC). A uniform external field (100 Oe or 500 Oe) is then applied and the induced magnetization is measured during warming the sample at a constant rate. Field cooling (FC) measurements proceed in a similar manner to ZFC except that the constant external field is applied while cooling and heating. However, the magnetization is usually measured while heating. The remanence (REM) blocking curve can easily be calculated as the difference between the FC and ZFC curve. For the magnetic measurements on Neocapritermes opacus, we used individuals that had undergone an iron deprived diet. The diet aimed at cleaning their guts of any ingested detrital material4, eliminating non-biogenic, soil-derived magnetite. Termites with extracted hemolymph were also studied. ZFC/FC curves show no Verwey transition and indicate a bimodal particle volume distribution. The derivative of the REM curves were fitted with two lognormal distributions, rendering 20 K and 90-180 K, respectively, as the mean blocking temperatures (intervals). Assuming magnetite as the constituent of the nanoparticles, the mean blocking temperatures correspond to mean volumes of 3,0*103 nm3 and 1,4-2,3*104 nm3. The lower value is in good agreement with those reported using ferromagnetic resonance spectrocopy5. The results are discussed and compared with those obtained on hemolymph and on particles eliminated with the diet. 1 Vácha, M. Biologia Bratislava 52 (1997) 629 2 Leal, I. and Oliveira, P.S.. Behav. Ecol. Sociobiol. 37 (1995) 373 3 Acosta-Avalos, D. et al. Naturwissenchaften 88 (2001) 343 4 . Maher, B.A. P Roy Soc Lond B Bio 265 (1998) 733 5. Alves et al J Magn Reson 168 (2004) 246